State laws that allow medical marijuana use, including the Michigan Medical Marihuana Program (MMMP), create policy tension due to a paucity of scientific data about factors that influence: marijuana use patterns, abuse potential, decisions to seek/renew medical certification, impact of marijuana use on HIV risk behaviors, and health outcomes. This proposal will address these knowledge gaps by studying marijuana-using persons living with HIV/AIDS (PLWHAs) in Michigan's largest HIV primary care clinic (Detroit, 1700 patients) with a prospective mixed-method approach, as described in these Specific Aims: Aim 1. In 120 marijuana-using PLWHAs (certified medical users, non-certified therapeutic users, and recreational users; 40/group), we will (A) use behavioral economic simulations to evaluate marijuana demand sensitivity to experimental manipulation of unit price and income; and (B) determine whether marijuana price elasticity, cross-price elasticity with other drugs/therapies, and income elasticity differ across marijuana user groups. Aim 2. In a subset of 36 marijuana-using PLWHAs (certified medical, non-certified therapeutic, and recreational users; 12/group from Aim 1), conduct a laboratory-based, double blind, placebo-controlled, marijuana smoking evaluation (cumulative 0, 4, 8 and 12 puffs from 3.5% delta 9-THC cigarettes). We will (A) determine whether marijuana dose-effect profiles for symptom relief and abuse liability differ between marijuana-using groups; (B) compare smoked marijuana demand across groups; and (C) compare intensity of experimental (gold standard) and naturalistic smoking effects. Aim 3. In the same 120 marijuana-using PLWHAs as Aim 1, use chart review, urinalysis, and interviews to measure prospectively at baseline, and at quarterly intervals for one year, clinical factors that are hypothesized to influence marijuana use and medical certification: socioeconomic changes, physical and mental health, substance use, criminal justice involvement, acute care utilization, HIV risk-transmission behaviors, and reasons for marijuana use. We will use clinical data alone and combined with simulated marijuana demand (Aim 1) and smoked marijuana responses (Aim 2) to predict longitudinal patterns of marijuana and alternative therapy use. Aim 4. Determine whether prevalence of certified medical marijuana use, non-certified therapeutic use, and recreational use in the 120-patient prospective sample generalizes to the entire HIV clinic. We will survey a random sample of 300 PLWHAs to (A) determine whether marijuana use patterns are representative within this large urban clinic located in the area with disproportionally high HIV prevalence, and (B) compare characteristics of this sample to prior cross-sectional survey data. At the conclusion of this project, we anticipate that (1) these data will reveal distinct patterns within this population; (2) we will have achieved a more sophisticated understanding of medical marijuana use, and (3) we will be able to predict which subsets of HIV patients are likely to benefit or be harmed. These findings will be disseminated to patients, providers and society to promote public health and informed policy debate.